1. Field of the Invention
The present invention relates to process oil which is to be added to natural or synthetic rubber and a process of producing the process oil, and in particular, to rubber process oil which can be easily handled and whose toxicity and carcinogenicity are low due to reduction of the content of polycyclic aromatics and the production process thereof.
2. Description of the Related Art
The process oil is used to facilitate the operation of manufacturing the rubber, such as kneading, extrusion, molding, etc., by means of its propertiy to penetrate into the texture of a rubber polymer. Moreover, it is also used to improve the physical properties of rubber products. For such process oil, it is necessary to have suitable affinity for rubber. On the other hand, the rubber to be processed can be classified into natural rubber and synthetic rubber, and there are many types of synthetic rubber. Among them, natural rubber and styrene-butadiene rubber (SBR) are used in specially large amounts. Accordingly, process oil containing a large amount of aromatic hydrocarbons, which have a high affinity for the rubber, is generally used.
To prepare this rubber process oil, use is made of a lubricant fraction obtained by distillation of crude oil under reduced pressure; an oil which is obtained by deasphalting residues distilled under reduced pressure, subjecting as necessary to dewaxing treatment and hydrogenation refining treatment, and further subjecting to sulfuric acid refining treatment; or an extract which is obtained by extraction treatment of oil with a solvent having an affinity for aromatic hydrocarbons. Thus obtained rubber process oil contains 70 to 99% aromatics as determined by column chromatography, has a % C.sub.A value of 20 to 50% as determined by ring analysis according to the Kurtz method, and contains 5 to 25% by mass of extracted matter with dimethylsulfoxide (DMSO) which corresponds to a content of polycyclic aromatics (PCAs) according to the standards of The Institue of Petroleum.
In recent years, however, the carcinogenicity of PCAs comes to be regarded as important. In Europe, oil etc. containing 3 or more % PCA are obliged to indicate that they are toxic, and there is a trend toward regulation of their use. Accordingly, it becomes an urgent task to reduce the content of PCAs in rubber process oil to less than 3%.
For the rubber process oil with the content of PCA reduced to less than 3%, Japanese Laid-Open Patent No. 6-505,524 (national publication of Japanese translation of Internatinal Patent Application No. PCT/US92/00379) discloses a process which comprises deasphalting treatment of a reduced-pressure residue fraction and then dewaxing treatment of the resulting oil in order to produce rubber process oil with a reduced content of PCA of less than 3%. Moreover, Japanese Laid-Open Patent No. 7-501,346 (national publication of International Patent Application No. PCT/US91/04497) discloses a non-carcinogenic bright stock extract and a deasphalted oil having a low PCA content as well as a process for producing the same, wherein use is made of an oil obtained by deasphalting of residues in a vacuum distillation column, or an oil whose aromatics are reduced by extraction treatment of the deasphalted oil, or an oil obtained by dewaxing treatment thereof.
The above oils have a lower content of PCA, but then have a high aniline point. The aniline point can indicate the content of aromatic hydrocarbons, and to have a high aniline point means a low content of aromatic hydrocarbons. However, according as the content of aromatic hydrocarbons in oil is decreased, the affinity of the oil for rubber is decreased. Therefore, there is still the problem that an essential property of rubber process oil, i.e. the ability of rubber process oil to blend a rubber polymer, is lowered in the materials disclosed in the above publications. Moreover, there is also the problem that it becomes difficult to satisfy the physical states of a final rubber product.